1. Field of the Invention
This invention relates to cleaning methods, and concerns in particular the removal of drilling fluid or "mud" from the annular space between the casing and the sides of the bore of an oil/gas well prior to cementing in.
The construction of a well, such as an oil well, involves the drilling of a borehole into the ground through the geological formations of interest. Drilling mud is used to lubricate and cool the drill bit, to assist in bringing the drill cuttings up to the surface, and to provide sufficient hydrostatic pressure within the well to prevent the bore collapsing under the forces experienced deep underground and to prevent the influx of fluids from the formation while drilling is taking place. A typical present-day water-based mud is an aqueous mixture of bentonite clays, carboxymethyl-cellulose, xanthan gum, barite and other constituents such as polymers etc.
2. Description of the Prior Art
Once the drilling is finished the bore must be cased and cemented, a procedure in which first the casing, having an external diameter a little smaller than the bore diameter, is placed in the mud-filled bore and then cement (as a fine slurry having a typical "generic" composition of cement and water, with small amounts of accelerator, retarder, fluid loss additive, dispersant, extender, weighting agent and various additives to control properties such as thixotropy, gel strength and foaming) is fed into the annulus between the casing and the borehole wall, and allowed to set to form a lining therefor. It is common procedure to supply this cement to the top of the casing, and then to pump it down the inside of the casing to the bottom and then up the outside of the casing, between the casing and the bore walls, until it fills and lines the annulus. In practice a loose-fitting "rubber"-like bung (with a frangible/rupturable centre section) is placed in the casing on top of the mud already therein, cement is then pumped in on top to provide a "plug"-like volume of cement calculated to be enough to fill the annulus, a second bung is placed on top of the cement, and then more mud is pumped in to force the bung-bounded plug of cement down to the bottom of the casing. When the lower plug reaches a restriction at the bottom of the casing an increase in applied pressure ruptures the bottom bung, and the cement is pumped out of the bottom of the casing and then back up to the top, but now on the outside, in the annulus. As it progresses up the annulus, so it fills up the space, seeping into all the cracks and fissures in the bore wall, and then setting to provide the desired sealing of the sides of the bore.
It is important that the cement lining be complete--that there be no places where the volume between casing and bore wall be empty of cement, and that the cement be well bonded both to the casing exterior and to the bore walls--and to ensure this it is of course necessary to sweep out the mud in the annulus ahead of the rising cement. Unfortunately, this often proves extremely difficult, and various procedures and mechanisms have been devised in the past to improve the chances of achieving efficient mud-removal and forming a complete cement lining. Indeed, mud removal is especially difficult in situations where the casing is not properly co-axial with the bore (where the eccentricity, or offset, of the casing in the bore is high, and conversely where on one side the "stand-off" of the casing from the bore is low), and where, as is increasingly the case these days, the mud is or includes one or more of the various polymeric ingredients which give it many desirable properties but which also can cause it to gel, and thus make it much more difficult to remove. Some of the techniques and equipment employed to assist in mud removal are: "mud conditioning", in which mud is circulated around the system for some time prior to pumping in the cement, in an effort to make more mobile any "congealed" mud already existing; "casing movement", in which the entire string of pipe constituting the casing is physically moved, by lifting/dropping and rotation, to break up any mud deposits; "casing centralisers", used when running in the casing in an attempt to prevent it being placed eccentrically; and "scratchers", which travel up and down with the casing, rotating as it is rotated, to scratch away gelled mud.
It is also possible, and indeed desirable, to employ, ahead of the cement, liquids that will literally wash the mud off the casing and bore walls. These can be pumped into the casing on top of the mud and before the cement, and may be of the type known as "chemical washes", usually low-viscosity liquids containing surfactants and mud thinners, or "spacers", rather more viscous, gel-like liquids that are primarily to form a buffer between the cement and the mud.
Unfortunately, although all these mud removal schemes do work, they cannot be guaranteed, especially where the casing eccentricity is high (so that the casing is in parts of its length touching, or almost touching, the side of the bore), and it is very common for the cement to fail properly to fill the annular space because of mud trapped immovably between casing and bore wall, so that there results a cement void. So common, indeed, is this that not only do around 50% of all bores fail in this way, and have to be rectified by an expensive operation known as "squeezing" (after locating the failure area, a hole is explosively blown in the casing at the appropriate spot, and cement is then pumped directly through the hole into the void), but as many as 30% fail in more than one place.
It will be evident that there is a considerable need for an effective way of removing the mud, especially gelled mud, from the annulus ahead of the cement, and the invention proposes a novel technique which is intended for this purpose.